The demand for improved surface coatings and powder particle products in the thermal spray industry has been relentless as the technology suffers from compositional non-homogeneity of injected solution precursors. One method to achieve homogeneity in coatings and particle products is aimed at repeatedly producing precursor droplets with uniform diameter. Precise control of the size of the solution droplets injected into a thermal spray system facilitates precise control of the particle melt for improved coating and powder generation. One general method for droplet generation using capillary streams involve the use of a piezoelectric device that impinges a pressure pulse on the walls of a reservoir vessel full of a liquid solution. In general, one such method is the imposition of amplitude modulated sinusoidal carrier disturbances on the piezoelectric device. These methods generally involve piezoelectric actuator(s) (“piezo”) in direct contact with a liquid source. One method involves using an oscillating crystal in direct contact with the liquid source to impart a disturbance and initiate capillary instability responsible to break up a single stream into uniform droplets. The disturbance is imposed in a compressive fashion at the top of the liquid volume and propagated downstream to a capillary nozzle. Another method imparts this disturbance on the side wall of a columnar liquid contained in a radially contracting piezoelectric cylinder that forces liquid through a capillary nozzle and is said to produce uniform stream of droplets. These droplet generation methods are, in general, limited to large droplet diameters and/or work at frequencies no higher than 10 KHz.
Applications of droplet apparatuses known in the art have the piezo in direct contact with the functional liquid. For example, in a typical printer design, the piezo is immersed in the printing liquid and serves as a gate to allow or forbid droplet exit as the piezo stretches or contracts under electrical drive. In another application, the piezo oscillations are transmitted directly to the liquid. In this application the piezo may be in direct contact with the liquid or, if not in direct contact, the transmission is done through an elastic membrane. Furthermore, the effect of oscillations affects only a small volume of liquid directly near the nozzle.